Congratulations! You have just
successfully completed your homebrew antenna project!
After all this effort, you are left with only one question: Was it worth the
effort? Does the antenna life up to your expectations? There are two ways of
finding out:

Not everyone has measurement equipment for testing
2.4GHz antennas laying around ;-) There are however some simple tests you can
perform to establish if an antenna behaves as an antenna, or an elaborate
air-cooled 'dummy-load'.

For the method described here, we make use of standard Radio LAN
equipment to assess your homebrew antenna. The test will give a fairly good idea
if the antenna functions as an antenna ... or not. For a more specific results,
sadly you will need more specialised equipment.

You will need an Access Point as an RF Signal generator and
a
Wifi client to determine signal levels. The Antenna Under Test (AUT) can be
connector to the
Access Point (or wireless router, as a transmit-antenna) or to the client (as receiving-antenna).
Antenna parameters have reciprocity – This means that an antennas parameters are
the same in both directions; transmitting and receiving. You can either connect
the Antenna Under Test to either an Access Point or Wifi Client to test it.

Connect the antenna with a minimal length of coaxial
cable. The cable must be suitable for the frequency and should have the lowest
loss possible. This minimises the effect of the coaxial cable on the measurement
results. Make sure that the connection is secure. Nothing is more frustrating
than an intermittent contact during measurement.

For a reliable measurement you must get a descent
distance between the antennas of the Access Point and Client. 10 to 20 meter
will do just fine.
Best results are obtained with measurements in free space. En open field is
ideal, but may not be practical. You may use an Access Point that is part of
somebody else's network, just as long as you have an idea about the physical
location of that AP.

In the following description, I assume that the AUT is
connected to the Wifi client, as shown in the picture above. An access Point is
used as a signal source, using its standard antenna.

You can choose either one of both options that follow
to perform the measurement.

Make use of the Signal Strength indication in the utility software
supplied with the card, or

If you use the utility software, connect your client
to the reference Access Point. Simply read the value from the "Signal Strength"
bar shown in most utility programs.

When you use
Netstumbler, this is not required. Simply start it up, and select the reference AP
from the list by double-clicking on it. The stronger the received signal, the
higher the green area will be.

The paragraph title says it all. Simply measure the
signal strength with and without an antenna connected to your Wifi client. The
must be a clearly noticeable difference. If signal level is low and connecting
the antenna has no effect, improve the alignment between the antennae on the Access Point
and the client. Also inspect the connecting cable, or try to connect another
(known good) antenna to it if you have one available. Last, reduce the distance
between the Access Point and the Client.

If signal level is high with and without the external
antenna connected, try increasing the distance between the two devices.

Directional antennas have an effect on radio waves like
a torchlight on light. Most energy is bundled in a beam at the front of the
antenna. To the side and rear of the antenna, the signal level is less. Turn the
antenna around the vertical axis. You must notice a clear difference in signal
strength during the rotation. Make sure not to shield (part of) the antenna with
your body.

Omni's (Omni directional Antennas) have no directivity.
This means that you should notice little or no variations in signal strength
when the antenna is rotated around its vertical axis.
You should however notice a change in signal strength when the antenna is tilted
towards the Access Point. If the signal strength increases, the antenna has what
is called an electrical 'uptilt'. If the signal strength increases when
the antenna is tilted away from the AP, the antenna has an electrical 'downtilt'.

Directional and Omni directional antennas both have a
parameter called polarisation. Antenna polarisation is defined as the direction
of the Electric field of the antenna, in reference to the earth's horizon. Most
commonly used are horizontal and vertical polarisation. A standard Wifi
equipment dipole antenna has vertical polarisation, when mounted vertically. An
antenna with vertical polarisation performs at his best in conjunction with
another vertically polarised antenna. When antennas with different polarisation
are mixed, the result is a decrease in signal strength - the so called
cross-polarisation rejection. This effect must be clearly present with your
antenna.

Rotate the antenna around the horizontal axis, keeping
the front of the antenna pointed toward the Access Point's Antenna. You must be
able to distinct a clear maximum and minimum in signal strength.

Another form of polarisation is found in the Helical or Helix antenna.
This antenna has a circular polarisation. The signal strength should remain
constant when this antenna is rotated around its bore sight.

If the antenna responds as expected in all three tests,
you can assume that your antenna indeed functions as an antenna. Whether or not
the antenna delivers the gain that you expect an only be estimated by comparing
the results of this antenna with those of an antenna with known gain. Maximum
signal strength received should be about equal for both antennas.

As indicated above, some programs/ cards indicate
signal strength in linear percent scale (0-86% in my case), whilst others
(Netstumbler) do the same in dBm, a logarithmic scale. In theory you are able to
determine the absolute gain of an antenna in dBi by comparing the signal
strength (in dBm) of two antennas. The difference in signal strength in dBm is
the same as the difference in gain in dBi between those two antennas. If you
have a signal; strength op -60dBm on a dipole antenna (2dBi gain) and a signal
strength of -50dBm using the Antenna Under Test, than the gain of the AUT
is 2dBi + (-50dBm - -60dBm) = 12dBi.

Unfortunately, you can not trust the indicated signal
strength to be absolutely true. The indicated signal strength in Netstumbler is
just an interpretation of the signal reported by the chipset in your Wifi card.
In most cases, the signal strength detector is not even a logarithmic scale, but
a linear one. This means that it is not possible to determine the
absolute gain of your antenna. It is however possible to determine the
relative gain of an antenna. i.e.: Is its gain more, loss or equal to that of
the reference antenna.

Relative Indicated Sign al Strength Indication (RSSI)
is also dependent on the chipset used in your equipment. You can not compare
Signal Strength between two different cards (even of the same type!) and expect
to gain meaningful results.

Thé most widely used antenna test equipment with
amateur antenna builders is the VSWR meter (Voltage Standing Wave Ratio meter).
Using an SWR meter an an RF Generator, it is possible to determine if an
antenna is correctly tuned to a frequency. A Homebrew VSWR meter, designed
specifically for use with Wifi equipment, can be found here.

The gain of an antenna can be determined on an antenna-range.
A Simplified antenna-range, and a protocol on how to perform the measurement
with the use of simple homebrew equipment, can be found on my
Practical Wifi antenna measurements page.

The Internet community "Wireless Nederland" organises a
yearly event called WOAF (Wifi Open Air Festival) where antenna gain is
measured using professional equipment.

I will not be responsible for damage to equipment, your ego, personal injury or worse that may result from the use of this material and material found on any links on my pages. You are responsible to make sure that your use any of my designs is legal in your country.